Flow-through self-cleaning gas oven with heat exchanger

ABSTRACT

A self-cleaning gas-fired oven having a power burner beneath the oven cooking cavity so the flue gases pass up through the cooking cavity and exit through an oven exhaust means at the top rear of the oven. A counterflow heat exchanger rising from the oven exhaust means, and an air collector means at the bottom of the heat exchanger to convey the preheated cooling air to the power burner by means of a motor-blower unit. The heat exchanger is of special design with cylindrical flue gas tubes so the opposite wall plates operate at relatively cool temperatures.

Hurko et al.

Jan. 8, 1974 FLOW-THROUGH SELF-CLEANING GAS OVEN WITH HEAT EXCHANGER Inventors: Bohdan Hurko; Raymond L. Dills, both of Louisville, Ky.

Assignee: General Electric Company Filed: Aug. 1, 1972 Appl. No.2 277,073

U.S. c1 .Q 126/21 R 1m. 01 A21b 1/2s Field of Search 126/19, 21, 273

References Cited UNITED STATES PATENTS Primary Examiner-Edward G. Favors AttorneyRichard L. Caslin et a].

[57] ABSTRACT A self-cleaning gas-fired oven having a power burner beneath the oven cooking cavity so the flue gases pass up through the cooking cavity and exit through an oven exhaust means at the top rear of the oven. A counterflow heat exchanger rising from the oven exhaust means, and an air collector means at the bottom of the heat exchanger to convey the preheated cooling air to the power burner by means of a motor-blower unit. The heat exchanger is of special design with cylindrical flue gas tubes so the opposite wall plates operate at relatively cool temperatures.

10 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION The present invention was developed while working toward improved designs of pyrolytic or high temperature self-cleaning gas-fired ovens wherein food soils and grease spatters lodged on the inner walls defining an oven cooking cavity are degraded into gaseous products that are completely oxidized before they are returned to the kitchen atmosphere. In such a selfcleaning oven the oven wall temperatures rise to somewhere between about 750F. and about 950F. and are held for a sufficient time to complete the transformation, on the order of 2 to 4 hours. In a gas-fired oven only about one half of the heating value of the gas consumed is used for heating the oven. The remainder is represented by both the heat of exhaust and the heat losses radiating outwardly from the walls of the oven body or cabinet. One difficult tehcnical problem present in a gas self-cleaning oven that is not found in an electric self-cleaning oven is the excessive amount of heat energy that is exhausted into the kitchen atmosphere as a function of both the high temperature and the large volume of flue gases.

Two earlier patents that are related to the present invention are first U.S. Pat. No. 3,507,265 of the present inventors entitled SELF-CLEANING GAS COOKING OVEN, and U.S. Pat. No. 3,659,646 also by the present inventors entitled HEAT EXCHANGER CONSTRUC- TION. Both of these patents disclose so-called muffle ovens where the self-cleaning oven cooking cavities are heated by passing convection currents of flue gases over the outside of the oven liner and then passing the gases up through a counterflow heat exchanger located behind the oven liner. 1

The principle object of the present invention is to provide a flow-through self-cleaning gas cooking oven with the most efficient counterflow heat exchanger.

A further object of the present invention is to provide a counterflow heat exchanger for a gas oven of the class described where the heat exchanger has as its main component a collapsed corrugated sheet metal panel that forms a generally flat closed wall plate with a plurality of vertical spaced cylindrical flue gas tubes, there being a second flat wall plate fastened across the tubes to form alternate cooling air channels between the gas tubes so that the outer surfaces of the heat exchanger operates at relatively cool temperatures.

A further object of the present invention is to provide a gas self-cleaning oven with a power burner means for improving the efficiency of combustion as well as improving the down flow of cooling air and the up flow of flue gases through the heat exchanger.

SUMMARY OF THE INVENTION The present invention, in accordance with one form thereof, relatesto a flow-through, self-cleaning gastired oven having an oven cooking cavity with a bake burner located beneath the cavity so that flue gases pass through the cavity and out through the oven exhaust means adjacent the top-rear of the oven cavity. A counterflow heat exchanger rises from the oven exhaust means and it has vertically arranged flue gas tubes and alternate cooling air channels. An air collector means is mounted to the bottom of the heat exchanger for joining the cooling air channels to the lower bake burner to supply pre-heated air to support combustion. Further improvements are made by the use of a heat exchanger construction of collapsed corrugated sheet metal design with cylindrical flue gas tubes so that the outer walls of the heat exchanger operate at relatively cool temperatures thereby reducing the amount of thermal insulation necessary to maintain relatively low exterior temperatures on the oven body. Another important improvement was realized in adopting a power burner in place of an atmospheric burner.

BRIEF DESCRIPTION OF THE DRAWINGS Our invention will be better understood from the following description taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended Claims.

FIG. 1 is a side elevational view in cross section of a free-standing gas range with a flow-through self cleaning oven and heat exchanger combination embodying the present invention.

FIG. 2 is a fragmentary, exploded perspective view on an enlarged scale of the counterflow heat exchanger of FIG. 1 showing the nature of the components and the method of assembling them.

FIG. 3 is a fragmentary perspective view of the tapered air collector means attached at the top to the bottom portion of the heat exchanger and tapering down to a small duct size adapted to be joined to the intake of the motor-blower unit of the power bake burner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to a consideration of the drawings and in particular to FIG. 1, there is shown a free-standing domestic gas range 10 comprising a sheet metal outer cabinet or range body 11 supporting a horizontal cooktop 13 at counter height and an underlying oven cooking cavity 15. The cooktop 13 supports a plurality of surface burners 17, but these are not illustrated in detail since the present invention centers around the oven construction. The two surface burners 17 that are shown are mounted on a single drip pan 19 having integral gas passageways which connect with individual gas valves 21 arranged along the front edge of the cooktop 13 on a common manifold that is in turn connected to a source of gas that connects into the back of the range as is conventional in this art. The surface burners are not illustrated in detail since they do not form part of the present invention. Arranged along the back of the cooktop 12 is a backsplash 23 which supports the oven controls 25 in the front control panel such as oven control valves, oven timers, meat thermometer dials, automatic surface burner temperature controls and electrical convenience outlets.

The oven cooking cavity 15 is formed by a box-like oven liner 27 in combination with a front-opening access door 29. The oven liner 27 has a removable bottom wall or panel 31, opposite vertical side walls 33, a rear wall 35, and a top wall 37. Normally the opposite side walls 33 would be provided with some type of oven rack support means such as embossed ledges (not shown) so that the racks may be installed at varying heights within the oven. Such food supporting racks are conventional and are not illustrated here for the sake of simplicity. A thick layer of thermal insulation 39 surrounds the vertical walls of the oven liner and also covers the top wall 37. Moreover, the oven door 29 is well insulated by a fiber glass or the like material 39. A fire box or combustion chamber 41 is located beneath the oven liner and access may be gained to the fire box either by removing the bottom plate 31 of the oven liner or by opening a small dummy drawer front or panel 43 in the front of the range beneath the oven door 29.

A bake burner 45 is supported within the fire box 41, and it is shown as a power burner that is supplied with slightly compressed air from a motor-blower unit 47 that is mounted adjacent the burner within the fire box. It should be understood however that a conventional atmospheric burner could be used in the present invention, but that the use of a power burner 45 is the preferred embodiment of the present invention since it results in improved combustion efficiency and therefore less fuel being used and less heat energy being expended as well as better overall operational characteristics. The power burner 45 included a plenum or mixing chamber 49 between the motor-blower unit 47 and the burner head 45. The conventional oven gas manifold, gas lines, solenoid valves and pilot burners or electric ignition means are not illustrated as they do not form part of the present invention, but are conventional components being used today in standard gas ranges. A heat spreader plate 51 is carried beneath the removable bottom panel 31 to protect this bottom panel from excessive heat which might otherwise injure the porcelain enamel finish that covers the interior walls forming the oven cooking cavity 15. Another layer 39 of thermal insulating material is arranged beneath the fire box 41 so as to protect from excessive temperatures the floor on which this range is to be supported.

The removable bottom wall 31 of the oven liner is provided with flue gas inlet openings 55, which are preferably larger at the front of the oven than at the back so that more hot flue gases pass near the oven door 29 so as to obtain generally uniform interior wall temperatures for uniform cooking and cleaning results due to the heat loss through and around the oven door 29. A high temperature door sealing gasket 57 such as woven fiber glass is furnished with the door to seal the door gap between the front flange of the oven liner and the inner surface of the door to restrict the amount of heat that leaks through the door gap as well as to restrict the amount of ambient air that will be allowed to enter the oven cooking cavity during the high temperature self-cleaning cycle. This oven door 29 is adapted to be latched and maintained shut during the high temperature cycle by a door latch mechanism 59 that is located in the front of the cooktop 13 just above the free edge of the door 29. A satisfactory oven door latch mechanism is described in the patent of Joseph S. Fox, Sr., US. Pat. No. 3,367,697, which is assigned to the same assignee as is the present invention.

An oven exhaust vent 65 is located adjacent the top of the oven cavity 15 preferably in the back wall 35 of the oven liner so that the hot flue gases that enter the cavity through the openings 55 in the removable panel 31 may pass throughout the oven and finally be exhausted through the vent 65. Rising upwardly from the oven vent 65 is a counterflow heat exchanger 67 which can best be seen in FIG. 2. This heat exchanger is of light weight sheet aluminum construction comprising a collapsed corrugated panel 69 and an opposite, generally flat wall plate 71 having wraparound opposing end walls 73. The corrugated panel 69 is collapsed in the plane thereof so as to form a generally flat closed wall plate 70 with a plurality of vertical, spaced cylindrical flue gas tubes 75. These tubes are of generally oval configuration and each tube has a butt joint 77 that is substantially closed due to pressure exerted from each side of the corrugated panel tending to collapse the panel, as is best seen in FIGS. 2 and 3. When the opposite wall plate 71 is assembled around the collapsed corrugated panel 69, the end walls 73 encompass the corrugated panel 69 and restrain the corrugations to the collapsed position as seen in FIG. 2. A metal tension strip 79 is wrapped around the assembly and clamped in place by a crimped fastener 81 so that the heat exchanger is a self-sustaining assembly. Thus it can be seen that the spaced flue gas tubes are separated by alternate cooling air channels 83 with an air channel 83 also arranged on each end of the heat exchanger.

The lower portion of the heat exchanger is fitted with an adaptor or manifold 85 which extends the complete width of the heat exchanger, and in the front it is fitted with the oven exhaust vent 65. The top wall of adaptor 85 is provided with holes 87 which are aligned with the flue gas tubes 75 such that the flue gases pass from the oven cavity 15 through the oven exhaust vent 65 and then through the holes 87 up through the flue gas tubes 75. These flue gases cannot enter the cooling air channels 83.

A horizontal adaptor 89 is fastened to the top of the heat exchanger, and it has a bottom wall 91 that is provided with a series of holes 93 which are also aligned with the flue gas tubes 75 for receiving the flue gases and passing them forwardly through the adaptor to exhaust through an elongated slot or grillwork 95 in the front of the adaptor. This top adaptor 89 has been fashioned into a secondary heat exchanger by installing parallel fins 97 to the underside of the top wall 99 of the adaptor. The purpose of these fins 97 is to pick up some of the heat in the flue gases and carry this heat through the top, wall 99 thereby creating a warming shelf for use during normal cooking operations so that food that has been cooked may be kept warm by placing the food on the secondary heat exchanger 89. Moreover, this warming shelf is useful durng the selfcleaning cycle in reducing the heat energy in the exhaust gases that are returned to the kitchen atmosphere by conducting some of the heat to the warming shelf 99.

Notice that the heat exchanger 67 extends up behind the backsplash 23 of the range 10, and that no blanket of insulation is shown between the heat exchanger and the backsplash. The reason that this can be done is that the outer surfaces of the heat exchanger 67 operate at a relatively cool temperature as compared to the temperature of the flue gases in the gas tubes 75. This is because there is a generally line contact between each tube 75 and the rear wall plate 71. Also the butt joint 77 of each tube 75 has only a double metal thickness joining the front side of each tube with the closed flat panel 70 of the collapsed corrugated plate 69. While 1 have elected as my preferred embodiment a cylindrical flue gas tube design 75 a generally oval transverse configuration, it will be understood by those skilled in this art that other similar forms or shapes can be used in the present invention, with the understanding that there should be little or a minimum amount of a heat conduction path between the tubes and the opposite panels 70 and 71. This of course is to maintain relatively cool temperatures on the outer walls of the heat exchanger as compared with the temperature of the flue gases. Looking at FIG. 1 a vertical layer of thermal insulation 39 is shown beneath the backsplash 23 at the rear edge of the cooktop 33. Moreover, there is another blanket of insulation 39 behind the heat exchanger 67.

Looking at FIG. 2, the upper portion of the rear wall plate 71 of the heat exchanger is provided with a horizontal series of holes 105 which communicate with the cooling air channels 83. An air manifold 107, shown in FIG. 1, is assembled over these holes 105, and this manifold is provided with a rear air inlet 109 so that room air is drawn from the back of the range into the air manifold 107 and passes through the holes 105 into the alternate cooling air channels 83 wherein the air passes down through the heat exchanger and takes on some of the heat present in the walls of the flue gas tubes 75 and thereby becoming heated at the same time lowering the temperature of the flue gases which of course is the main purpose of employing the heat exchanger 67.

The bottom portion of the rear wall plate 71 is also provided with another series of holes 111 which also communicate with the cooling air channels 83, and a second air manifold 113 extends the width of the heat exchanger and fits over these holes for receiving the heated air from the bottom of the heat exchanger.

An air collector means 117 is arranged behind the oven liner.27, and it is adapted to connect through the air manifold 113 to the bottom portion of the heat exchanger 67 and to carry this heated air down to the fire box 41 where this heated air is delivered to the motorblower unit 47. The air collector means 117 is shown as of downwardly tapered configuration, as is best seen in FIG. 3, where it is drawn down into a small chamber 119 having a circular opening 121 on its front side for receiving a cylindrical duct 123 that connects into the intake of the blower 47 Notice that a blanket of insulation 39 is arranged behind the air collector means 117 as well as between the air collector means and the back wall 35 of the oven liner 27.

Modifications of this invention will occur to those skilled in this art, therefore, it is to be understood that this invention is not limited to the particular embodiments disclosed, but that it is intended to cover all modifications and their equivalents which are within the true spirit and scope of this invention as claimed.

What is claimed as new and is desired to be secured by Letters Patent of the United States is:

l. A gas oven comprising a substantially box-like oven liner having a front opening, and an oven door cooperating with the opening for defining an oven cooking cavity, a fire box located beneath the oven liner, gas burner means positioned in the fire box, flue gas inlet openings in the bottom wall of the oven liner for passing the flue gases into the cooking cavity, oven exhaust means positioned adjacent the back of the oven liner near the top thereof, and a counterflow heat exchanger at the back of the oven rising up from the oven exhaust means, the heat exchanger having both flue gas vent means and ambient air inlet means at the top of the heat exchanger so that the ambient air extracts much of the heat from the flue gases as they pass through the heat exchanger, and a collector means attached to the bottom of the heat exchanger and communicating with the burner means so that the ambient air flowing down through the heat exchanger becomes heated and passes to the gas burner means as preheated air to, support combustion.

2. A gas oven as recited in claim 1 wherein the said counterflow heat exchanger comprises a collapsed corrugated sheet metal panel forming a generally flat closed wall plate with a plurality of vertical spaced cylindrical flue gas tubes on the innermost side of the wall plate, and a generally flat wall plate held across the cylindrical tubes thereby forming alternate coolingair channels between the gas tubes, whereby the flue gases are adapted to pass by convection up the cylindrical tubes and the ambient air is adaptedto pass down through the cooling air channels, whereby the outer wall plates operate at relatively cool temperatures during the operation of the heat exchanger as compared with the temperature of the flue gases.

3. A gas oven as recited in claim 2 wherein the said gas burner means is furnished with a compressed air supply means for supplying fresh combustion air to the burner means, said air supply means being connected to the said air collector means beneath the heat exchanger to draw the ambient air down through the heat exchanger as preheated air before delivering it to the gas burner means.

4. A gas oven as recited in claim 3 wherein the said air supply means is a centrifugal motor-blower unit located adjacent the gas burner means beneath the oven liner.

5. A pyrolytic self-cleaning gasfired oven comprising wall means defining an oven cooking cavity, a bake burner at the bottom of the oven cavity, an exhaust flue in the said wall means adjacent the upper rear of the cavity for discharging flue gases from said oven cavity, and a counterflow heat exchanger rising up from the exhaust flue and located generally above the oven cooking cavity so as to create a strong natural draft up through the heat exchanger and avoid the necessity of a circulation fan, and an oven control panel located in front of the heat exchanger, an air inlet means adjacent the top of the heat exchanger, air collector means hehind the oven joined to the bottom portion of the heat exchanger and open to the bake burner means to furnish preheated air to the burner to improve the efficiency of combustion, the top of the heat exchanger having flue gas vent means directed outwardly away from the back of the oven with cooling means in conjunction with a warming shelf to serve as a secondary heat exchanger.

6. A pyrolytic self-cleaning gas-fired oven as recited in claim 5 wherein the said bake burner means is a powered burner that is, furnished with a forced air supply means, the said air collector means from the bottom of the heat exchanger being connected to the forced air supply means, so that the air is both drawn down the heat exchanger and compressed and then supplied to the burner means.

7. A flow-through, self-cleaning gas-fired oven comprising an oven cooking cavity formed by a box-like oven liner and a front opening access door, a bake burnerlocated beneath the bottom wall of the oven liner, flue gas inlet openings in the said bottom wall for passing the flue gases into the cooking cavity, oven exhaust means adjacent the top rear of the oven liner, and a counterflow heat exchanger with vertically arranged flue gas tubes and alternate cooling air channels extending for substantially the width of the oven liner, the

- flue gas tubes being connected at the bottom to the said oven exhaust means and having flue gas vent means adjacent the top of the heat exchanger that are directed away from the back of the oven, the air channels having inlet means adjacent the top of the heat exchanger, and.

an air collector means mounted on the bottom portion of the heat exchanger and joined to the cooling air channels and cooperating with the bake burner to supply preheated air to support combustion.

8. A flow-through, self-cleaning gas-fired oven as recited in claim 7 wherein the said bake burner is a power burner that is furnished with a motor-blower means interposed between the air collector means and the power burner to both accelerate the velocity of the air cooling means passing down through the heat exchanger and compress this preheated air before delivering it to the power burner so as to improve the efficiency of combustion and thereby lower the amount of heat energy needed to cook as well as to decompose the food soil lodged on the walls forming the oven cooking cavity.

9. A flow-through, self-cleaning gas-fired oven as recited in claim 7 wherein the said counterflow heat exchanger comprises a collapsed corrugated sheet metal panel forming a generally flat closed wall plate with a plurality of spaced cylindrical flue gas tubes on the innermostside of the wall plate, and a generally flat wall plate held across the cylindrical tubes with a generally line contact with each, thereby forming alternate cooling air channels between the gas tubes, whereby the exterior walls of the heat exchanger operate at relatively cool temperatures during the operation of the heat exchanger as compared with the temperature of the flue gases.

10. A flow-through, self-cleaning gas-fired oven as recited in claim 9 wherein the said bake burner is a power burner that is furnished with a motor-blower means interposed between the air collector means and the power burner to both accelerate the velocity of the air cooling means passing down through the heat exchanger and compress this preheated air before delivering it to the power burner so as to improve the efficiency of combustion and thereby lower the amount of heat energy needed to cook as well as to decompose the food soil lodged on the walls forming the oven cooking cavity. 

1. A gas oven comprising a substantially box-like oven liner having a front opening, and an oven door cooperating with the opening for defining an oven cooking cavity, a fire box located beneath the oven liner, gas burner means positioned in the fire box, flue gas inlet openings in the bottom wall of the oven liner for passing the flue gases into the cooking cavity, oven exhaust means positioned adjacent the back of the oven liner near the top thereof, and a counterflow heat exchanger at the back of the oven rising up from the oven exhaust means, the heat exchanger having both flue gas vent means and ambient air inlet means at the top of the heat exchanger so that the ambient air extracts much of the heat from the flue gases as they pass through the heat exchanger, and a collector means attached to the bottom of the heat exchanger and communicating with the burner means so that the ambient air flowing down through the heat exchanger becomes heated and passes to the gas burner means as preheated air to support combustion.
 2. A gas oven as recited in claim 1 wherein the said counterflow heat exchanger comprises a collapsed corrugated sheet metal panel forming a generally flat closed wall plate with a plurality of vertical spaced cylindrical flue gas tubes on the innermost side of the wall plate, and a generally flat wall plate held across the cylindrical tubes thereby forming alternate cooling air channels between the gas tubes, whereby the flue gases are adapted to pass by convection up the cylindrical tubes and the ambient air is adapted to pass down through the cooling air channels, whereby the outer wall plates operate at relatively cool temperatures during the operation of the heat exchanger as compared with the temperature of the flue gases.
 3. A gas oven as recited in claim 2 wherein the said gas burner means is furnished with a compressed air supply means for supplying fresh combustion air to the burner means, said air supply means being connected to the said air collector means beneath the heat exchanger to draw the ambient air down through the heat exchanger as preheated air before delivering it to the gas burner means.
 4. A gas oven as recited in claim 3 wherein the said air supply means is a centrifugal motor-blower unit located adjacent the gas burner means beneath the oven liner.
 5. A pyrolytic self-cleaning gas-fired oven comprising wall means defining an oven cooking cavity, a bake burner at the bottom of the oven cavity, an exhaust flue in the said wall means adjacent the upper rear of the cavity for discharging flue gases from said oven cavity, and a counterflow heat exchanger rising up from the exhaust flue and loCated generally above the oven cooking cavity so as to create a strong natural draft up through the heat exchanger and avoid the necessity of a circulation fan, and an oven control panel located in front of the heat exchanger, an air inlet means adjacent the top of the heat exchanger, air collector means behind the oven joined to the bottom portion of the heat exchanger and open to the bake burner means to furnish preheated air to the burner to improve the efficiency of combustion, the top of the heat exchanger having flue gas vent means directed outwardly away from the back of the oven with cooling means in conjunction with a warming shelf to serve as a secondary heat exchanger.
 6. A pyrolytic self-cleaning gas-fired oven as recited in claim 5 wherein the said bake burner means is a powered burner that is furnished with a forced air supply means, the said air collector means from the bottom of the heat exchanger being connected to the forced air supply means, so that the air is both drawn down the heat exchanger and compressed and then supplied to the burner means.
 7. A flow-through, self-cleaning gas-fired oven comprising an oven cooking cavity formed by a box-like oven liner and a front opening access door, a bake burner located beneath the bottom wall of the oven liner, flue gas inlet openings in the said bottom wall for passing the flue gases into the cooking cavity, oven exhaust means adjacent the top rear of the oven liner, and a counterflow heat exchanger with vertically arranged flue gas tubes and alternate cooling air channels extending for substantially the width of the oven liner, the flue gas tubes being connected at the bottom to the said oven exhaust means and having flue gas vent means adjacent the top of the heat exchanger that are directed away from the back of the oven, the air channels having inlet means adjacent the top of the heat exchanger, and an air collector means mounted on the bottom portion of the heat exchanger and joined to the cooling air channels and cooperating with the bake burner to supply preheated air to support combustion.
 8. A flow-through, self-cleaning gas-fired oven as recited in claim 7 wherein the said bake burner is a power burner that is furnished with a motor-blower means interposed between the air collector means and the power burner to both accelerate the velocity of the air cooling means passing down through the heat exchanger and compress this preheated air before delivering it to the power burner so as to improve the efficiency of combustion and thereby lower the amount of heat energy needed to cook as well as to decompose the food soil lodged on the walls forming the oven cooking cavity.
 9. A flow-through, self-cleaning gas-fired oven as recited in claim 7 wherein the said counterflow heat exchanger comprises a collapsed corrugated sheet metal panel forming a generally flat closed wall plate with a plurality of spaced cylindrical flue gas tubes on the innermost side of the wall plate, and a generally flat wall plate held across the cylindrical tubes with a generally line contact with each, thereby forming alternate cooling air channels between the gas tubes, whereby the exterior walls of the heat exchanger operate at relatively cool temperatures during the operation of the heat exchanger as compared with the temperature of the flue gases.
 10. A flow-through, self-cleaning gas-fired oven as recited in claim 9 wherein the said bake burner is a power burner that is furnished with a motor-blower means interposed between the air collector means and the power burner to both accelerate the velocity of the air cooling means passing down through the heat exchanger and compress this preheated air before delivering it to the power burner so as to improve the efficiency of combustion and thereby lower the amount of heat energy needed to cook as well as to decompose the food soil lodged on the walls forming the oven cooking cavity. 